What is moon day number 0?

Month days are counted from 1 to 30/31 or 28/29 for February. Can anyone explain a situation with numbering of moondays? And what is day number 0? What is the last number?I downloaded a set of moonphases which got files numbered from 1 to 31. Files 1, 2, 30 and 31 are just blank. I started showing files on my app according to ageOfMoon param and soon found that Wunderground has ageOfMoon zero. Should I always add 1 to ageOfMoon to show the right moon icon? Thanks.